Freeze on loading screen

Finally bought Garry's Mod and tried playing it today, but the game freezes on the loading screen you get just before the menu appears. At first I thought it was just the mod doing something in the background, but decided to kill it after nothing happened for half an hour. Any ideas?

The game also just started randomly downloading a whole gig of extra stuff just now. Quite strange since I have HL2 installed (heck, it got automatically downloaded along with the mod).